Refining benzene with a sodium and potassium metal alloy



United States Patent 3,096,382 REFINING BENZENE WITH A SODIUM AND POTASSIUM METAL ALLOY Oscar L. Wright, McKces Rocks, John Wilson Mausteller, Evans City, and Frank J. Vaucheri, Pittsburgh, Pa., assignors to MSA Research Corporation, Pittsburgh, Pa. No Drawing. Filed Apr. 13, 1960, Ser. No. 21,866 6 Claims. (Cl. 260-674) This invention relates to the purification of hydrocarbons, and more specifically to the removal of thiophene and other impurities from benzene and benzene fractions by treatment with an alloy of alkali metals.

The main commercial source of benzene is coal; it is formed by the pyrolysis operation known as coking. A reasonably pure benzene fraction containing some other aromatic hydrocarbons, organic sulfur compounds, organic nitrogen compounds, and phenols is separated from coal gas by well-known methods. Separation and purification methods have been developed and are generally used which successfully remove all of the denaturants except thiophene; coke plant benzene generally contains 100 ppm. or more thiophene. Benzene is also obtained from petroleum distillate fractions by a process of aromatization. The benzene produced by this method is very pure, but generally contains -10 p.p.m. of thiophene.

For many purposes it is necessary or desirable to use benzene that is substantially thiophene free. The catalytic hydrogenation of benzene, for example, requires the use of catalysts which are poisoned by small amounts of thinphene or other sulfur compounds.

A number of methotds of purifying benzene and benzene fractions have been developed which remove some thiophene, or can remove substantially all of the thiophene under special conditions. The most used method is the acid wash process in which the benzene is intimately contacted with concentrated sulfuric acid. The contaminants are decomposed or polymerized and are removed with the acid sludge. In order to obtain the desirable very low concentration of thiophene, however, a large amount of sulfuric acid is required, and the process results in substantial losses of benzene and the accumulation of large amounts of useless acid sludge. Other methods such as hydrogenation or reaction with sodium at the necessary high temperature, e.g., about 250 C., require high pressures with the resultant expense and hazard.

It is therefore an object of this invention to provide a simple, direct, inexpensive methotd of purifying benzene and benzene fractions. A further object is to provide a method of removing thiophene from benzene and benzene fractions that does not require high temperatures or pressures. Another object is to provide a method of removing thiophene from benzene or benzene fractions that minimizes benzene losses. A still further object is to provide a method of purifying benzene or benzene fractions containing thiophene impurity by treatment with an alloy of alkali metals. Other objects will become apparent from the following specification and claims.

This invention is based on our discovery that alloys of sodium and potassium rapidly and completely remove thiophene from benzene or benzene fractions at low temperatures with the formation of solid products that are easily separable from the benzene.

According to this invention, benzene or a benzene fraction containing thiophene impurity is contacted with an alloy of sodium and potassium in a batch or continuous manner at a temperature as low as about 50 C. The alloy readily reacts with the thiophene impurity to form a precipitate which is easily separated by decantation, filtration, distillation and other conventional methods. The alloy also reacts with other impurities such as carbon disulfide, thioethers, Water, carbon dioxide and the like, to form solid, easily separable products.

3,096,382 Patented July 2, 1963 Any alloy of sodium and potassium is suitable for use in this invention, and it is preferred to use an alloy that is liquid at ambient temperature since it is more conveniently handled, more easily brought into intimate contact with the benzene or benzene fraction, and more adaptable to continuous processing methods. Alloys of sodium and potassium are commercially available at moderate cost. The sodium-potassium alloys that are fluid at normal room temperatures contain between about 35 and 90 potassium. Neither sodium nor potassium alone are suitable, however, since they do not remove a detectable amount of thiophene even at temperatures substantially above their melting point.

The following examples illustrate the remarkable effect of the use of alloys of alkali metals. Benzene (500 cc.) containing 300 ppm. thiophene and 2.0 grams of 56% K 14% Na alloy were charged to a 1 liter Morton flask equipped with an agitator, and reflux condenser. The mixture was agitated at reflux temperature for one hour, and the benzene was then distilled and analyzed for thiophene. Substantially all the thiophene was removed, and the refined benzene contained less than 1 ppm. thiophene. Similar results are obtained using alloys containing from about 1 to 99% potassium. However, when 2.6 g. of molten potassium and 500 cc. of benzene containing 300 ppm. thiophene were contacted under identical conditions no thiophene was removed. Similarly, 10.0 grams of sodium dispersion (50% in xylene) contacted with 500 cc. of benzene under identical conditions removed no detectable amount of thiophene.

The method of this invention may be used equally well to purify benzene in which thiophene is the only substantial impurity and benzene fractions which contain other impurities. If the benzene fractions contain large amounts of other impurities which can easily be separated by distillation, it is generally more economical to at least partially remove them by distillation before treatment with sodium-potassium alloy. To illustrate, most of the water was removed by azeotropic distillation from an acidwashed light oil having a composition of about 81% benzene, 13% toluene, 2% xylene, 2% other hydrocarbons, 2% H 0 and 275 p.p.rn. thiophene. The light oil was reacted with 10.0 g. of 56% Na-44% K alloy (alloy/thicphene ratio of /1) at reflux temperature for 15 minutes, and the resultant light oil contained only 0.3 ppm. thiophene. This product can be used directly in applications that are not adversely affected by other hydrocarbons, or the mixture may be distilled to separate pure thiophene-free benzene. The removal of two percent Water by reaction consumes about 15 grams of alloy, so that about 25 g. of alloy is required to purify 500 cc. of the crude acid-washed light oil. It is generally not economical or desirable to remove small or trace amounts of distillable impurities from benzene fractions before purification by the method of this invention, because their removal by reaction consumes only a small amount of alloy.

About 1.5 parts by weight of alloy are consumed in removing 1 part of thiophene, so at least about 1.5 parts of alloy for each part of thiophene are required to essentially remove all the thiophene. The exact mechanism of the reaction is not known precisely, but virtually all of the sulfur appears in the reaction product as metal sulfides. The organic fragment undoubtedly polymerizes, because organic tars are found in the organic residue. Carbon-ates and hydroxides are generally also found in the residue, reflecting the almost universal presence of Water and carbon dioxide in benzene. Additional alloy is consumed by other purification reactions when the henzene contains impurities other than thiophene. There is essentially no loss of benzene due to reaction with the alloy.

3 It is preferred to use at least about 3 parts alloy to each part thiophene to accomplish rapid removal of the thiophene and provide alloy for reaction with other trace impurities. Even larger excesses of alloy may be used, if desired, to promote the rate of purification. This is illustrated by the results of reactions set forth in Table I.

TABLE I Efiect of NaK/Tlzz'ophene Ratio on Rate Removal at Reflux Temperature 1 Purified benzene containing about 0.4 to 0.7 ppm. thlophenc.

1 Couunerclul nitration grade benzene. When large excesses of the alloy are used to promote reaction rate, only a small portion of the alloy is actually consumed by the purification reaction, and the excess is readily recoverable for reuse.

It is generally preferred to use a temperature between about 60 and 80 C., but higher or lower temperatures may be used if desired. At lower temperatures the rate of reaction is slower, and below about 50 C. there is substantially no reaction. At higher temperatures superatmospheric pressure is developed by the benzene, and pressure equipment is required. The following reactions illustrate the effect of temperature on the rate of reaction. Benzene containing thiophene (210 ppm. thiophene) and sufficient 56% K-44% Na alloy to give a 90-100:1 alloy to thiophene ratio were contacted in a Morton flask. After minutes contact at the desired temperature, the benzene was rapidly distilled from the mixture and analyzed for thiophene content. Thirteen percent of the thiophene was removed at 50 C.; 30.6% removed at 60 C.; 52.5% removed at 68 C.; and 99.9% removed at 70 C. and higher temperatures. Thus longer reaction times are required at lower temperatures for complete removal of the thiophene, e.g. a period of several hours is required at 50 C.

The method of the invention is particularly adaptable to continuous methods, as is illustrated by the following example. A l-liter Morton flask with suitable inlet and outlet connections and equipped with a 4000 rpm. dispersing impellor was used as a reactor. A charge of 500 m1. of benzene containing 200300 p.p.m. thiophene and 10 g. of 56% K-44% Na alloy was heated to reflux and agitated. Forty-two liters of the contaminated benzene (average thiophene content 265 ppm.) was continuously added to the flask at a rate of two liters per hour, and purified benzene Was continuously distilled from the flask. An additional 0.5 g. of alloy was added with each liter of benzene. Each liter of the purified benzene was sampled and analyzed for thiophene content; the thiophcne content ranged from 0.5 to 1.3 ppm. and averaged 1 ppm. for the entire 42 liters. The 42 grams of residue sludge separated by filtration contained sulfides, hydroxides, and carbonates of the alkali metals, organic polymers, and about 10.5 g. of unreacted alloy. The presence of the sludge is not detrimental to the purification reaction, so that it can be removed in any convenient manner at any time.

Other methods of performing a continuous reaction will be apparent to those familiar with the art. For example, impure benzene and alloy may be continuously fed to a reactor maintained below the reflux temperature, or at a higher temperature under pressure, so that benzene is not distilled from the reactor. The reaction mixture can then be passed to a filter separating the sludge from the purified benzene. The benzene is readily separated by distillation from any excess or unreacted alloy that passes the filter.

It is generally desirable to use vigorous agitation when carrying out the purification reaction to promote contact of the alloy and the benzene. High speed propellors or specially designed dispersing impellors are especially desirable. Although increased agitation increases reaction rate, practical reaction rates can be obtained with moderate mixing, as with a paddle-type agitator, when moderately high proportions of alloy are used; e.g. about 40 parts per part of thiophene. And with even higher proportions of alloy, e.g. 100 parts per part of thiophene, agitation caused by boiling when the reaction is run at eflux temperature has been found adequate.

According to the provisions of the patent statutes, we have explained the principle and mode of practicing our invention, and have described what we now consider to be its best embodiment. However, it is understood that within the scope of the following claims the invention may be practiced otherwise than as specifically described.

We claim:

1. A method of purifying benzene that contains thiophene impurity which comprises contacting said benzene in the liquid phase with an alloy of sodium and potassium at a temperature about C. and 80 C., using at least about 1.5 parts alloy for each part of thiophcne to be removed, and recovering the purified benzene.

2. A method according to claim 1 in which the temperature is between about and the atmospheric pressure reflux temperature.

3. A method according to claim 1 in which the benzene and alloy are contacted with vigorous agitation.

4. A method according to claim 1 in which the alloy contains between about 35 and 90% potassium.

5. A method of purifying benzene that contains thiophene impurity which comprises continuously feeding said benzene and an alloy of sodium and potassium to a contacting chamber, contacting said alloy and said benzene in the liquid phase at a temperature between about 50 C. and C., using at least about 1.5 parts alloy for each part of thiophenc to be removed, and continuously recovering the resultant purified benzene.

6. A method according to claim 5 in which the alloy contains between about 35 and potassium.

References Cited in the file of this patent UNITED STATES PATENTS 1,859,028 Cross May 17, 1932 1,939,839 Williams et al. Dec. 19, 1933 2,772,211 Hawkes et al Nov. 27, 1956 2,927,074 Berger et al. Mar. 1, 1960 2.960,546 Nobis et a] Nov. 15, 1960 2,979,548 Clarke Apr. 11, 1961 

1. A METHOD OF PURIFYING BENZENE THAT CONTAINS THIOPHENE IMPURITY WHICH COMPRISES CONTACTING SAID BENZENE IN THE LIQUID PHASE WITH AN ALLOY OF SODIUM AND POTASSIUM AT A TEMPERATURE ABOUT 50*C. AND 80*C., USING AT LEAST ABOUT 1.5 PARTS ALLOY FOR EACH PRT OF THIOPHENE TO BE REMOVED, AND RECOVERING THE PURIFIED BENZENE. 